Proton exchange membrane materials based on sulfonated poly (phthalazinones)

ABSTRACT

A novel class of proton exchange membrane materials, sulfonated poly(phthalazinones), were prepared by modification of poly(phthalazinone). Sulfonation reactions were conducted at room temperature using mixtures of 95-98% concentrated sulfuric acid and 27-33% fuming sulfuric acid with different acid ratios in order to get SPPEK with degree of sulfonation (DS) in the range of 0.6 to 1.0. The presence of sulfonic acid groups was confirmed by FT-IR analysis, and the DS and structures were characterized by NMR. The introduction of sulfonic groups into the polymer chains led to an increase in the glass transition temperature and a decrease in the decomposition temperature. Membrane films were cast from solution in N,N-dimethylformamide or N,N-dimethylacetamide. Water uptakes and swelling ratios of the membrane films increased with DS and sulfonated polymers with DS&gt;1.2 were water soluble at 80° C. Proton conductivity increased with DS and temperature up to 95° C., reaching 10 −2 S/Cm.

BACKGROUND OF THE INVENTION

In recent years, polymer electrolyte fuel cells have been identified aspromising power sources for vehicular transportation and otherapplications requiring clean, quiet and efficient portable power. As avital part of the fuel cell, proton exchange membranes (PEM)s havebecome a rapidly growing area of research. Until now, PEMs have beenmainly limited to perfluorinated ionomer membranes such as Nafion™developed by DuPont and similar membranes commercialized by Dow andAsahi. In spite of their outstanding properties such as excellent protonconductivity and oxidative resistance, which are essential for fuel cellapplication, the perfluorinated ionomer membranes are very expensive(US$800-2000/m²) and suffer from other serious drawbacks of highmethanol permeation and dehydration. In order to develop alternatives toNafion™, which would be less expensive and free from other disadvantagesof perfluorinated ionomer membranes, several attempts have been recentlymade including synthesis of new polymer electrolytes¹⁻⁵, chemicalmodification of available high performance polymers⁶⁻¹⁰, and blendmembranes¹¹⁻¹⁴. Among these studies, introducing sulfonic acid groupsinto the main chains of high performance polymers by sulfonationreaction is an important and widely used method for imparting polymerswith proton exchange capability. Poly(aryl ether ketone)s,poly(phenylene oxide), poly(phenylene sulfide), poly(aryl ethersulfone), and polybenzimidazole are among those that have been studied.The sulfonating agents include concentrated sulfuric acid,chlorosulfonic acid, pure or complex sulfur trioxide, and acetylsulfate. As an example, the sulfonation reaction of Victrex™ PEEK andthe conductivity of sulfonated PEEK have been studiedextensively^(7,11,15-17). The DS could be controlled by reaction timeand temperature in concentrated sulfuric acid or oleum. For sulfonatedPEEK with a relatively low DS of 0.65, its conductivity reaches 0.04 Scm⁻¹ at 100° C./100% RH, higher than that of Nafion-117 measured underthe same conditions.

Poly(phthalazinone ether ketone) (PPEK) is one of a new class ofpoly(aryl ether ketone)s under consideration for commercialization. PPEKhas a very high glass transition temperature of 263° C., excellenthigh-temperature stability, and many other good properties. Thesulfonation reactions of its copolymer, poly(phthalazinone ether sulfoneketone) (PPESK), and the nanofltration and ultrafiltration membraneproperties have also been studied¹⁸⁻²³.

SUMMARY OF THE INVENTION

According to the invention, the sulfonation reaction of severalpoly(pthalazinones) have been investigated, and suitable PEMs acquiredfrom the sulfonated products.

According to one aspect of the invention, a series of sulfonatedpoly(pthalazinone)ether ketones (PPEKs), (SPPEKs) with different DSswere prepared via modification of PPEK with the mixture of concentratedsulfuric acid and fuming sulfuric acid as the sulfonating agent. Thestructure and some properties of SPPEKs and resulting membrane filmswere characterized, including: sulfonation position, protonconductivity, equilibrium water uptake, swelling ratio andthermostability as a function of DS.

Similarly, a series of poly(pthalazinone) ether sulfone ketones(PPESKs),and poly(pthalazinone) ether sulfones(PPESs) were prepared and tested.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of the synthetic pathway for SPPEK;

FIG. 2 is a ¹H-NMR spectrum of PPEK in CDCl₃;

FIG. 3 is a ¹H-NMR spectrum of SPPEK in DMSO-d₆;

FIG. 4 is a ¹³C-NMR spectrum (hydrogen decoupled) of PPEK in CDCl₃;

FIG. 5 is a FT-IR spectra of PPEK and SPPEK;

FIG. 6 is a graph illustrating TGA traces of PPEK and SPPEKS;

FIG. 7 is a graph illustrating the Conductivity of SPPEK;

FIG. 8 is a graph of Conductivity versus temperature for SPPES andSPPESK; and

FIG. 9 is a graph of Conductivity of SPPEKs of various DS.

DETAILED DESCRIPTION OF THE INVENTION EXPERIMENTAL

Materials

PPEK was synthesized according to the procedure reportedpreviouslyls^(18,19). All other chemicals obtained commercially werereagent grade and used as received.

Sulfonation Reaction

In a typical small-scale experiment, 0.5 g PPEK powder was added to a 10mL mixture of 95-98% concentrate sulfuric acid and 27-33% fumingsulfuric acid under an argon atmosphere and the mixture was magneticallystirred at room temperature (23° C.) for a certain reaction time. Thereported reaction time is the total time for polymer dissolution andreaction. After a determined reaction time the reaction medium waspoured onto crushed ice and the resulting precipitate was recovered byfiltration, washed with deionized water until the pH value was ˜6-7.

For scaled-up reactions of 30-35 g, an ice bath was needed initially tocool the heat released during dissolution. An amount of 30 g PPEK powderwas added to a mixture of 240 mL 95-98% concentrated sulfuric acid and360 mL 27-33% fuming sulfuric acid under an argon atmosphere and themixture was magnetically stirred in an ice bath. About 0.5 h later, theice bath was removed and the stirring was continued at room temperature(23° C,). ¹H-NMR was used to trace the DSs. After a determined reactiontime, the reaction medium was poured onto crushed ice and the resultingprecipitate was recovered by filtration, washed with deionized wateruntil the pH value was ˜6-7.

Polymer Analysis and Measurement

Proton and carbon spectra were obtained on a Varian Unity Inova NMRspectrometer operating at a proton frequency of 399.951 MHz and a carbonfrequency of 100.578 MHz. Tetramethylsilane was used as the internalstandard chemical shift reference. ¹H-NMR spectra of PPEK and low DSSPPEK were acquired at a temperature of 22° C. in deuterated chloroform(CDCl₃). Deuterated methylene chloride (CD₂Cl₂) was used when accurateintegration values were desired for high field aromatic signals sincethe signal of residual CH₂Cl₂ (5.31 ppm) did not interfere with thearomatic region. Deuterated dimethylsulfoxide (DMSO-d₆) was the NMRsolvent of choice for higher DS SPPEK. Presaturation of the large waterpeak always present in SPPEK at around 4 ppm improved the spectra byincreasing the signal to noise ratio and by removing interferencesignals from the large water absorption. ¹³C NMR spectra of PPEK wereacquired using a 10 mm tunable broadband probe and a concentrated samplein CDCl₃ (500 mg in 3mL). A quantitative spectrum was obtained usinglong relaxation delay and hydrogen decoupling only during theacquisition time. IR spectra were measured on a Nicolet 520 Fouriertransform spectrometer with powder samples inside a diamond cell.

A TA Instruments thermogravimetric analyser (TGA) instrument model 2950was used for measuring the degradation temperatures (T_(d)) and a TAInstruments differential scanning calorimeter (DSC) model 2920calibrated with Tin at 231.93° C. and Zinc at 419.53° C. was used formeasuring the glass transition temperatures (T_(g)). Polymer samples forTGA analysis were preheated to 150° C. at 10° C./min under nitrogenatmosphere, held isothermally for 60 min, equilibrated at 80° C., thenheated to 800° C. at 10° C./min for T_(d) measurement. Hence, thedegradation data reported here were assumed to be in the absence ofmoisture. Samples for DSC analysis were initially heated at a rate of10° C./min under nitrogen atmosphere to well below the polymer T_(d)point, quenched in liquid nitrogen and then re-heated at the same rate.

Inherent viscosities were determined using an Ubbelohde viscometer forN,N-dimethylformamide solutions of polymer with a concentration of 0.5g/dL at 30° C.

Preparation of Membrane Films

An amount of 0.7 g sample was dissolved in 12 mL ofN,N-dimethylforniamide (DMF) or N,N-dimethylacetamide (DMAc) andfiltered. The filtered solution was poured onto a glass plate and driedat 40° C. for about two days. Residual solvent was further evaporated at120° C. under vacuum for 48 h, resulting in yellow membrane films.

Water Uptake Content Measurement and Swelling Ratio

All polymer membranes used were vacuum dried at 120° C. before test. Thesample films were soaked in deionized water for different time atdetermined temperatures. Weights of dry and wet membranes were measured.The water uptake content was calculated by${{Uptake}\quad{content}\quad(\%)} = {\frac{\omega_{wet} - \omega_{dry}}{\omega_{dry}} \times 100\%}$Where ω_(dry) and ω_(wet) are the masses of dried and wet samplesrespectively. The swelling ratio was calculated from films 7˜10 cm longby:${{Swelling}\quad{Ratio}\quad(\%)} = {\frac{l_{wet} - l_{dry}}{l_{dry}} \times 100\%}$Where l_(dry) and l_(wet) are the lengths of dry and wet samplesrespectively.Conductivity

The proton conductivity was measured by AC impedance spectroscopy over afrequency range of 1-10⁷ Hz with oscillating voltage 50-500 mV, using asystem based on a Solarton 1260 gain phase analyzer. A sample withdiameter 10 mm was placed in an open, temperature controlled cell, whereit was clamped between two blocking stainless steel electrodes with apermanent pressure of about 3 kg/cm². Specimens were soaked in deionizedwater prior to the test. The conductivity (a) of the samples in thetransverse direction was calculated from the impedance data, using therelation σ=d/RS where d and S are the thickness and face area of thesample respectively and R was derived from the low intersect of the highfrequency semi-circle on a complex impedance plane with the Re (Z) axis.

Results and Discussion

Sulfonation Reaction

FIG. 1 shows the sulfonation reaction, whereby sulfonation occurs asexpected around the electron-donating ether linkage.

In general, sulfonation of polymers can be conducted with severalsulfonating agents, including concentrated sulfuric acid, fumingsulfuric acid, chlorosulfonic acid and acetyl sulfate, depending on thereactivity of the polymer. For Victrex PEEK. a high DS can be achievedwith concentrated sulfuric acid because of the high reactivity of thehydroquinone segment in the polymer chain. We initially attempted thesulfonation of PPEK in 95-98% concentrated sulfuric acid at both roomand elevated temperatures. However, the results listed in Table 1 showthat almost no sulfonation of PPEK took place at room temperature evenfor a very long reaction time, and only SPPEK with a very low DS wasobtained at 60° C. after 60 h. Sulfonation is an electrophilic reactionaffected by both the electrophilicity of the sulfonating agent and theelectron donating characteristics of polymer. Compared with PEEK, PPEK(as shown in FIG. 1) has more electron-withdrawing functionality perrepeat unit, which decreases the reactivity of the electron-donatingether linkage of the polymer. In order to prepare SPPEK with a high DS,27-33% fuming sulfuric acid and elevated reaction temperature wereemployed. As shown in Table 1, the sulfonation of PPEK in oleum at 40°C. for 1 h resulted in SPPEK with a DS of 1.6. The DS didn't increasesignificantly over extended reaction times. This is because the sulfonicacid groups are more than 1.5 per repeating unit, reducing theelectron-donating characteristic of the polymer and preventing thefurther introduction of more sulfonic acid groups. Although a high DSwas achieved using fuming sulfuric acid, the reaction proceeded toorapidly to maintain control. Abating the reaction to room temperaturereduced the DS to 1.3. However, at a DS of 1.3, SPPEK is water solubleand can not be used as a PEM in fuel cell where high humidity exists. Afurther reduction in temperature made the PPEK solution in fumingsulfuric acid too viscous to completely dissolve the polymer, which mayresult in a heterogeneous sulfonation reaction. In order to reduce thereactivity of the sulfonating agent, a mixture of 95-98% concentratedsulfuric acid and 27-33% fuming sulfuric acid was used for PPEK. As seenin Table 1, by varying the ratio of concentrated sulfuric acid to fumingsulfuric acid and the reaction time, the sulfonation reaction was morereadily controlled to different DSs in the range of 0.6 to 1.23. TABLE 1Dependence of DS on the reaction conditions Oleum/Concentrated Reactiontemperature Reaction time sulfuric acid (° C.) (h) DS  0/10 R.T. 95  ˜060 6 <0.1 4/6 R.T. 4 0.1 5/5 R.T. 1 0.6 4 0.8 7 0.9 6/4 R.T. 1 0.8 4 1.07 1.1 7/3 R.T. 1 1.0 4 1.2 10/0  R.T. 1 1.3 40 1 1.6 2 1.6 23  1.7Reaction conditions: PPEK 0.5 g, mixture of sulfuric acid 10 mL, argonatmosphere. R.T.: Room TemperatureNMR

In order to determine the sulfonation site and the DS quantitatively,the ¹H-NMR spectra of PPEK in CDCl₃ (FIG. 2) and SPPEK in DMSO-D₅ (FIG.3) were characterized. The number sysem adopted for the protons in thepolymer and the derivatives are shown in FIG. 2 to 4. The careful andaccurate assignments of NMR spectral signals was essential fordetermining both DS and the site of sulfonation.

The starting point for peak assignment of ¹H and ¹³C-NMR of PPEK wasfrom the absorptions resulting from atoms surrounding the electron richether moiety. In a PPEK repeat unit, the hydrogen atoms at the orthoposition to the ether linkage are more shielded than any other hydrogenatoms due to resonance effect of the oxygen lone pair electrons. Theother functional groups present in PPEK have a deshielding effect onnearby nuclei. As a result of that, the furthest upfield signals(7.08-7.30 ppm) arise from the 4H ortho to the ether linkage. 2D COSYNMR as well as simple homonuclear decoupling experiments showed thepresence of two separate spin coupling systems originating from thesehigh field signals (FIG. 2). These interactions are a result ofspin-spin coupling between the hydrogen atoms at the ortho and metapositions of these phenol rings. One spin system consists of the highfield ortho ether 2H absorption at 7.16-7.30 ppm coupled with thedistinct signal at 7.60-7.72 ppm. The other spin system consists of thehigh field ortho ether 2H absorption at 7.08-7.16 ppm coupled with themultiple absorptions at 7.76-8.06 ppm.

The results of a simple ¹³C-NMR experiment allowed formal assignment ofthe preceding hydrogen signals, which are key components for theaccurate measurement of the DS of SPPEKs. The H-decoupled ¹³C-NMRspectrum of PPEK is displayed in FIG. 4. The carbon peaks of particularinterest are the ones arising from carbon atoms directly linked to anelectron-withdrawing heteroatom (N or O) causing absorptions to appearat lower field (144-195 ppm). The furthest downfield signal (193.6-194.8ppm) is unequivocally from the ketone carbon. Further upfield, threecarbon signals (155-162 ppm) arise from carbon atoms linked toelectronegative oxygen atoms in the phthalazinone and etherfunctionalities. Finally, the less electronegative nitrogen atoms givethe carbon absorptions at higher field (144-148 ppm). In one ¹³C-NMRexperiment, a spectrum was acquired with decoupling of one of thehydrogen frequencies (7.60-7.72 ppm) responsible for the 2H at the metaposition of one of the phenol rings. The resulting spectrum was comparedwith a ¹³C-NMR spectrum acquired with full hydrogen coupling. Of all thedownfield carbon signals, the C—O absorption at 156.0-157.1 ppm and theC—N at 147.0-148.0 ppm both lost a 6-7 Hz long range (3 bond C—C—C—H)carbon-hydrogen coupling when the hydrogen frequency 7.60-7.72 ppm hadbeen irradiated. Hence, the long-range C—H interaction could onlyoriginate from C(O)-27 coupled with H-25,29 and C(N)4 coupled withH-25,29. Decoupling of the other meta-ether hydrogen atoms H-19,23 wouldnot affect any of the 2 C-N carbons. This experiment lead to theunequivocal assignment of H-20,22 and H-26,28 which are essential forthe DS calculations. The assignment of H-25,29 and H-19,23 were alsoderived from this experiment. Further ID and 2D heteronuclear (C and H)NMR experiments confirmed the previous assignments. Table 2 and 3 listthe chemical shift of the ¹H and ¹³C-NMR spectra of PPEK repectively.TABLE 2 ¹H-NMR data of PPEK in CDCl₃ Chemical shift δ Integral intensityProton Number (ppm) (number of H) H-6,7,8,9,12,13,15,19,23 7.76-8.06 9HH-16 8.54-8.68 1H H-20,22 7.08-7.16 2H H-25,29 7.60-7.72 2H H-26,287.16-7.30 2H

TABLE 3 ¹³C-NMR data of PPEK (hydrogen decoupled) in CDCl₃. Chemicalshift δ Carbon Number (ppm) C-1 158.72 C-4 147.0-148.0C-5-10,12-16,18,19,23-25,29 125.0-137.0 C-11 144.8-146.0 C-17193.6-194.8 C-20,22 117.6-118.6 C-21 160.2-161.4 C-26,28 119.4-120.4 C27156.0-157.1

The hydrogen absorption at low field (8.54-8.68 ppm) is stronglybelieved to arise from H-16 (FIG. 2) although it could not be formallyproved. The chemical shift position of many peaks in ¹H and ¹³C-NMRspectra is seen to be not averaged out as would be the case where freerotation occurs. It is believed that PPEK is hindered by restrictedrotation around certain linkages. In a “restricted” structure, H-16would have an intensity of 1, as observed, and also would be in closeproximity to the phthalazinone carbonyl. The proximity to the carbonylwould deshield H-16 more than any other hydrogen atoms nearby one of thecarbonyl groups of PPEK.

DS Calculations From ¹H-NMR:

A ¹H-NMR of PPEK in CD₂Cl₂ was acquired and the integration value of theupfield H-26,28 and H-20,22 absorptions was set to 4.00. The integrationvalues of the other regions of the spectra corresponded exactly (table2) to the number of hydrogen atoms expected from the chemical structureof PPEK repeat unit. Similarly, in a quantitative ¹³C-NMR experiment,the upfield carbon signals C-26,28 and C-20,22 were also set to anintensity of 4.00 and the integration values of the other peaks againmatched precisely. Sulfonated PPEK is only soluble in highly polarsolvents and DMSO-d₄ was chosen to dissolve SPPEK. Spectra of SPPEK DS1.03 and 1.63 are displayed in FIG. 3. In comparison with PPEK, severalnew signals for the SPPEK derivatives appear with different chemicalshifts due to the different solvents used and perturbation by differentDSs of —SO₃H groups. The ortho-ether hydrogen atoms remained at highfield but their intensity decreased as they were replaced by —SO₃Hgroups. More importantly, the ratio of low field H-16 (8.40-8.55) ppm tomultiple peaks 7.60-8.40 ppm (which do not include ortho-ether protons)always remained 1.00:11.00 for low and high sulfonation degrees. This isproof that substitution occurred only at the ortho-ether sites of thePPEK repeat unit. Another phenomenon supporting this is the variation insize of the small high-field signal at 7.30-7.40 ppm due tomono-substitution on SPPEK repeat units. When strong electronwithdrawing sulfonic acid groups are attached to benzene rings, theyinduce deshielding of hydrogen in the ortho and para positions. Inmonosubstituted SPPEK (DS-1), 11-20 is deshielded by the —SO₃H grouphence shifted downfield. In disubstituted SPEEK repeat units (DS>1),H-20 is still deshielded by the —SO₃H group present on the phenol ringbut it is also shielded by the proximity through space of theelectron-rich oxygen atoms of the other —SO₃H group nearby on the otherphenol ring. The DS was simply measured by presetting the integrationvalue of the low field hydrogen absorptions to 12H (7.50-8.60 ppm) anddetermining the intensity value of the upfield hydrogen signals. Thisvalue represents the number of hydrogen atoms that have not beenconverted into —SO₃H groups and therefore substracting it from 4H(unmodified PPEK) gives a direct DS value for SPPEK.

FT-IR

Fourier Transform Infrared (FT-IR) Spectroscopy was used to confirm thependant SO₃H group on the polymer chain. FIG. 5 shows the FT-IR spectraof parent PPEK and its sulfonated derivatives with DSs of 1.03 and 1.63respectively. In comparing these spectra, one can see that in additionto the predicable absorptions at 3400 cm⁻¹ due to the stretching of thehydroxyls of SO₃H groups, the SPPEK absorption bands at 1020 and 1081cm⁻¹ are characteristic of the aromatic SO₃H symmetric and asymmetricstretching vibrations respectively. These two characteristic peaksincrease with higher DS. In addition, the reaction can be readilyfollowed by the signal at 1500 cm⁻¹ related to 1,4-aromatic ringsubstitution. Introduction of SO₃H onto the aromatic ring induces theformation of two new adsorptions at 1471 and 1475 cm⁻¹, which at highersulfonation degree, completely replace the adsorption at 1500 cm⁻¹. Theintroduction of sulfonic acid groups in the modified polymer is thusconfirmed.

Thermal Analysis

The thermal stabilities of the SPPEKs were determined by TGA. All thesamples were preheated at 150° C. for 60 min in the TGA furnace toremove moisture,then dynamic TGA experiments were run from 80 to 800° C.at a heating rate of 10° C./min under nitrogen. FIG. 5 shows thedegradation curves. The parent PPEK is a thermostable polymer of whichthe 5 wt % loss temperature is nearly 500° C. For the sulfonated PPEK,there are two transitions of loss in weight. The first one occurs atabout 300° C. and could be ascribed to the decomposition of the SO₃Hgroups. In order to confirm our speculation, the weight loss from theinitial point to 455° C. of SPPEK with DS of 1.0 was analyzed. A valueof about 15% was obtained, which is close to the theoretical SO₃H weightloss percentage of 16% in SPPEK with DS of 1.0. The second thermaldegradation at about 490° C. is assigned to the degradation of the mainpolymer chain, which is in close agreement to the weight loss step inthe TGA curve for parent PPEK. In addition, SPPEKs with higher DS loseweight more quickly than those with lower DS, in the temperature rangeof 300 to 460° C. It also indicates that the weight loss during thisperiod is due to the elimination of —SO₃H groups.

The T_(g) of starting material is 263° C. For sulfonated product, onlythe T_(g)S of SPPEKs with DSs of 0.1 and 0.6 have been detected at 270°C. and 292° C., respectively. The introduction of sulfonic groups intopolymer chains leads to increased T_(g)S because of the increasedintermolecular ionic interactions. For SPPEK with DS higher than 0.6, noT_(g)S have been detected because the decomposition temperatures arelower than the T_(g)S.

Solubility and Viscosity

PPEK is soluble in CHCl₃, chlorobenzene and some other chlorinatedsolvents, but insoluble in polar aprotic solvents such asN,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc) anddimethylsulfoxide (DMSO). SPPEK with low DS, such as DS of 0.1 hassimilar solubility characteristics to the parent polymer. However,SPPEKs with high DSs are soluble in these dipolar aprotic solvents, butinsoluble in chlorinated solvents. DMF was chosen as the solvent fordetermining the inherent viscosities [η] of SPPEKs with high DS, whichare listed in Table 4. It shows that the inherent viscosities of SPPEKsare higher compared with that of PPEK of 0.6 in chloroform and generallyincrease with increasing DS suggesting that the polymer chain is notdegraded during sulfonation. Only SPPEK with DS of 1.63 which isobtained with 27-33% fuming sulfuric acid had a slightly reducedviscosity. A significant increase in the viscosity may arise from twofactors: one is that the introduction of sulfonic acid into polymerchain caused an overall increase in the polymer molecular weight; theother is that hydrogen bonding related to sulfonic acid groups increasesthe molecular forces. The slight decrease in the viscosity of SPPEK withDS of 1.63 may be caused by degradation taking place during thesulfonation reaction in undiluted fuming sulfuric acid. TABLE 4 Inherentviscosities of SPPEK [η] DS dL/g 0 ^(a)0.6   0.6 1.40 0.8 1.81 1.0 1.831.2 1.99 1.6 1.88Determination conditions: SPPEK 0.5 g/dL in DMF, 30° C.^(a)In chloroformWater Uptake and Swelling Ratio

Adequate hydration of membranes is critical to fuel cell application.Water assists in the transportation of protons from the anode to thecathode. If the electrolyte membrane is too dry, its conductivity falls;on the other hand, excess water results in cathode flooding andmorphological instability of membrane. The water uptake and swellingratio of SPPEK membranes were determined by measuring the change in themass and length before and after hydration. The results are listed inTable 5, which show that the water uptake of SPPEK increases with the DSat both room temperature and elevated temperature. At 80° C., wateruptake and swelling ratio of SPPEKs with lower DS reach equilibriumvalues quickly. However, water uptake and swelling ratio of SPPEK withDS≧1 increase with time and some samples were swollen or dissolved inwater. The molecular structure of SPPEK is composed a hydrophobicbackbone and hydrophilic sulfonic acid groups. Absorbed water acts alsoas plasticizer, which promotes the phase separation. When the DS is arehigh enough, it becomes easy for the SPPEK chains to be separated bywater and dissolved. TABLE 5 Water uptake and swelling ratio of SPPEK80° C. Room temperature 2 h 24 h 3 days Water Swelling Water SwellingWater uptake uptake ratio uptake ratio DS (%) (%) (%) (%) (%) 0.1  6 142.4 14 2.4 0.6 15 16 3.6 16 3.6 0.8 19 21 6.4 22 6.4 1.0 19 44 12 63 201.2 32 Swelling Dissolved — — 1.6 100  Dissolved — — —Proton Conductivity

Prior to conductivity measurements, all membrane samples were soaked inwater 1 or 2 days for hydration. The effect of the DS on theconductivity of SPPEK is shown in FIG. 6, which shows that theconductivity of SPPEK at room temperature increases with DS and reaches2×10⁻² S/cm for SPPEK with DS of 1.2. This value is similar to Nafion117, which shows conductivity of 3×10⁻² S/cm. FIG. 7 also shows theinfluence of temperature on the conductivity for DS 1.0 and 1.2 SPPEKs.As can be seen, the conductivities of these two SPPEKs increase withincreasing temperature up to 95° C. and reach 4×10⁻² S/cm and 6×10⁻²S/cm, respectively. Conductivity of SPPEK with DS of 1.0 drops sharplyafter that, which is probably caused by the dehydration of membrane.Compared with Nafion 117, the drop in conductivity at 80° C. occurs at ahigher temperature for SPPEK.

Conclusions

A series of SPPEKs with different DS were prepared from PPEK with amixtures of fuming and concentrated sulfuric acid as both the solventand sulfonating agent. The Structure of SPPEK was confirmed by FT-IR andthe DS of SPPEK was determined by ¹H-NMR. As the DS of SPPEK increases,T_(d) decreased and T_(g) increased. Membrane films prepared from SPPEKsshow a continuous increase in water uptake and swelling ratio with DS.Membranes prepared form SPPEK with DS of 1.0 and 1.2 show attractivelyhigh conductivity of 10⁻² S/cm at both room temperature and elevatedtemperature.

Data for SPPES and SPPESK Prepared by Sulfonating PPES and SPPESKRespectively

Sulfonation Reaction of SPPES

An amount of 2 g PPES powder was added to a mixture of 16 mL 95-98%concentrated sulfuric acid and 24 mL 27-33% fuming sulfuric acid underan argon atmosphere and the mixture was magnetically stirred in an icebath. About 15 min later, the ice bath was removed and the stirring wascontinued at room temperature (23° C.). ¹H-NMR was used to trace theDSs. After a determined reaction time, the reaction medium was pouredonto crushed ice and the resulting precipitate was recovered byfiltration, washed with deionized water until the pH value was ˜6-7.

Sulfonation Reaction of SPPESK

An amount of 2 g PPESK powder was added to a mixture of 16 mL 95-98%concentrated sulfuric acid and 24 mL 27-33% fuming sulfuric acid underan argon atmosphere and the mixture was magnetically stirred in an icebath. About 15 min later, the ice bath was removed and the stirring wascontinued at room temperature (23° C.). ¹H-NMR was used to trace theDSs. After a determined reaction time, the reaction medium was pouredonto crushed ice and the resulting precipitate was recovered byfiltration, washed with deionized water until the pH value was ˜6-7.

Conductivity

Prior to conductivity measurements, all membrane samples were soaked inwater 1 or 2 days for hydration. FIG. 8 shows the influence oftemperature on the conductivity for SPPES and SPPESK with DS 1.0. As canbe seen, the conductivity of SPPESK increases with increasingtemperature up to 95° C. and reaches 2×10⁻² S/cm, then drops sharplyafter that, which is probably caused by the dehydration of membrane. Theconductivity of SPPES increases with increasing temperature up to 106°C. and reaches 4×10⁻² S/cm, then drops Compared with Nafion 117, thedrop in conductivity at 80° C. occurs at a higher temperature for SPPEK.

Proton Conductivity

Prior to conductivity measurements, all membrane samples were soaked inwater 1 or 2 days for hydration. The effect of the DS on theconductivity of SPPEK is shown in FIG. 9, which shows that theconductivity of SPPEK at room temperature increases with DS and reaches2×10⁻² S/cm for SPPEK with DS of 1.2. This value is similar to Nafion117, which shows conductivity of 3×10⁻² S/cm. FIG. 9 also shows theinfluence of temperature on the conductivity for DS 1.0 and 1.2 SPPEKs.As can be seen, the conductivities of these two SPPEKs increase withincreasing temperature up to 95° C. and reach 4×10⁻² S/cm and 6×10⁻²S/cm, respectively. Conductivity of SPPEK with DS of 1.0 drops sharplyafter that, which is probably caused by the dehydration of membrane.Compared with Nafion 117, the drop in conductivity at 80° C. occurs at ahigher temperature for SPPEK.

Data for SPPEK Prepared by Polymerization of Sulfonated Monomers

(This demonstrates that there are alternative methods of producingpoly(phthalazinones)

The random sufonated poly(phthalzinone ethers) have also been obtainedvia direct polymerization reactions with the sulfonation sites on thedeactivated aromatic rings as depicted in scheme 1.

Polymerization Reaction

The SPPEKs synthesized with various compositions are denoted as SPPEK-n,where n means the percentage content of SDFK in feed of DFK and SDFK.The synthesis of SPPEK-50 is used as a typical example. To a three-neckflask with a magnetic stirrer, a Dean-Stark trap and condenser, and anArgon inlet, 1.102 g DFK (5.05 nmol), 2.133 g SDFK-Na (5.05 mmol), 2.383g DHPZ (10 mmol), and 1.8 g potassium carbonate (13 mmol) were added.Then 18 mL of NMP and 25 mL chlorobenzene were charged into the reactionflask under an argon atmosphere. The reaction mixture was heated to 140°C. Upon dehydration and removal of chlorobenzene, the reactiontemperature was increased to 170-175° C. After a period of 5-7 h, whenthe solution viscosity had obviously increased, several milliliters ofNMP was added to dilute the solution and kept it for further 3-5 h.Then, the mixture was cooled to 100° C. and coagulated in ethanol, wateror acetone. After recovering and drying the product, SPPEKs werepurified by dialysis for 7 days, using a membrane-cellulose dialysistube (SPECTRUM) with a molecular weight cut off value of 3500.

Preparation of Membrane Films

An amount of 1 g SPPEK in sodium form was dissolved in 12 mL ofN,N-dimethylacetamide (DMAc) and filtered. The filtered solution waspoured onto a glass plate and dried at 40° C. for about two days.Residue solvent was further evaporated at 120° C. under vacuum for 48 h.resulting in yellow membrane films. The acid form membrane films wereobtained by doping the sodium form membrane films into 2 N H₂SO₄ for 48h, followed by doping in deionized water for 48 h, and vacuum drying at100° C. for 24 h.

REFERENCES

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1 though
 14. (canceled)
 15. Sulfonated poly(phthalazinones) ofstructural formula I


16. Sulfonated poly(phthalazinones) of structural formula I as definedin claim 15, in the form of a membrane.
 17. Sulfonatedpoly(phthalazinones) of structural formula I as defined in claim 15,wherein the degree of sulfonation (Ds) is in the range of 0.6 to 1.0.18. A process for the preparation of sulfonated poly(phthalazinones) ofstructural formula I as defined in claim 15, comprising reactingpoly(phthalazinones) of formula II

with a sulfonating agent
 19. A process according to claim 18, whereinthe sulfonating agent is a mixture of concentrated sulfuric acid andfuming sulfuric acid.
 20. A process according to claim 18, wherein thesulfonating agent is a mixture of 95-98% concentrated sulfuric acid and27-33% fuming sulfuric acid with different acid ratios.
 21. A processaccording to claim 19, wherein the sulfonating agent is a mixture of95-98% concentrated sulfuric acid and 27-33% fuming sulfuric acid withdifferent acid ratios.
 22. A process according to claim 19, wherein thedegree of sulfonation (DS) is controlled by varying the ratio ofconcentrated sulfuric acid to fuming sulfuric acid and the reactiontime.
 23. A process according to claim 20, wherein the degree ofsulfonation (DS) is controlled by varying the ratio of concentratedsulfuric acid to fuming sulfuric acid and the reaction time.
 24. Aprocess according to claim 21, wherein the degree of sulfonation (DS) isin the range of 0.6 to 1.23.
 25. A process according to claim 18,including the additional step of casting the sulfonatedpoly(phthalazinones) to form a membrane.
 26. A process according toclaim 19, including the additional step of casting the sulfonatedpoly(phthalazinones) to form a membrane.
 27. A process according toclaim 20, including the additional step of casting the sulfonatedpoly(phthalazinones) to form a membrane.
 28. A process according toclaim 21, including the additional step of casting the sulfonatedpoly(phthalazinones) to form a membrane.
 29. A process according toclaim 22, including the additional step of casting the sulfonatedpoly(phthalazinones) to form a membrane.
 30. A process for preparingsulfonated poly(phthalazinone) ether sulfone ketones, comprisingreacting a poly(phthalazinone) ether sulfone, with a sulfonating agent.31. A process according to claim 25, wherein the sulfonating agent is amixture of concentrated sulfuric acid and fuming sulfuric acid.
 32. Aprocess according to claim 26, wherein the sulfonating agent is amixture of concentrated sulfuric acid and fuming sulfuric acid.
 33. Amembrane electrode assembly for use in a fuel cell comprising: (a) ananode, (b) a cathode; and (c) a solid polymer electrolyte membranebetween said anode and said cathode, said solid polymer electrolytemembrane comprising a sulfonated poly(phthalazinone) of structuredformula I as defined in claim 15.